Antimocrobial agents An antimicrobial is an agent that kills microorganisms or inhibits their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibacterials (commonly known as antibiotics) are used against bacteria and antifungals are used against fungi.

Definitions of Antibiotics OLD: An antibiotic is a chemical substance produced by various species of microorganisms that is capable in small concentrations of inhibiting the growth of other microorganisms. NEW: An antibiotic is a product produced by a microorganism or a similar substance produced wholly or partially by chemical synthesis, which in low concentrations, inhibits the growth of other microorganisms. What is the role of antibiotic in nature: Germ Warfare- the production of compounds that discourage microbial competitors. Problem with this explanation is that antibiotic production by microbes growing in nature is so low that levels of antibiotics are undetectable. May be signaling molecules NO EVIDENCE FOR EITHER ROLE

* There is no perfect drug. The Ideal Drug* Selective toxicity: against target pathogen but not against host Bactericidal vs. bacteriostatic Favorable pharmacokinetics: reach target site in body with effective concentration Spectrum of activity: broad vs. narrow Lack of “side effects” Little resistance development Selective toxicity - greater harm to microbes than host, done by interfering with essential biological processes common in bacteria but not human cells. LD50 vs. MIC - Therapeutic index (the lowest dose toxic to the patient divided by the dose typically used for therapy). High TI are less toxic to the patient. Bactericidal vs. bacteriostatic Static rely on normal host defences to kill or eliminate the patogen after its growth has been inhibited. (UTIs) CIDAL given when host defenses cannot be relied on to remove or destroy pathogen. Favorable pharmacokinetics - drug interxns, how drug is distributed, metabolized and excreted in body (unstabel in acid, can it cross the Blood-brain barrier, etc) Spectrum of activity broad spectrum - wide Narrow spectrum - narrow range (pathogen must be ID’d) Lack of “side effects” allergic, toxic side effects, suppress normal flora Little resistance development * There is no perfect drug.

Antibacterial spectrum—Range of activity of an antimicrobial against bacteria. A broad-spectrum antibacterial drug can inhibit a wide variety of gram-positive and gram-negative bacteria, whereas a narrow-spectrum drug is active only against a limited variety of bacteria. Bacteriostatic activity—-The level of antimicro-bial activity that inhibits the growth of an organism. This is determined in vitro by testing a standardized concentration of organisms against a series of antimicrobial dilutions. The lowest concentration that inhibits the growth of the organism is referred to as the minimum inhibitory concentration (MIC). Bactericidal activity—The level of antimicrobial activity that kills the test organism. This is determined in vitro by exposing a standardized concentration of organisms to a series of antimicrobial dilutions. The lowest concentration that kills 99.9% of the population is referred to as the minimum bactericidal concentration (MBC).

Antibiotic combinations—Combinations of antibiotics that may be used (1) to broaden the antibacterial spectrum for empiric therapy or the treatment of polymicrobial infections, (2) to prevent the emergence of resistant organisms during therapy, and (3) to achieve a synergistic killing effect. Antibiotic synergism—Combinations of two antibiotics that have enhanced bactericidal activity when tested together compared with the activity of each antibiotic. Antibiotic antagonism—Combination of antibiotics in which the activity of one antibiotic interferes With the activity of the other (e.g., the sum of the activity is less than the activity of the individual drugs).

Beta-lactamase—An enzyme that hydrolyzes the beta-lactam ring in the beta-lactam class of antibiotics, thus inactivating the antibiotic. The enzymes specific for penicillins and cephalosporins are the penicillinases and cephalosporinases, respectively.

Susceptibility Tests 1. Broth dilution - MIC test Agar dilution, not used much, pour plates, tilt sideways 2. Agar dilution - MIC test

Minimal Inhibitory Concentration (MIC) vs. Minimal Bactericidal Concentration (MBC) 32 ug/ml 16 ug/ml 8 ug/ml 4 ug/ml 2 ug/ml 1 ug/ml Sub-culture to agar medium MIC = 8 ug/ml MBC = 16 ug/ml

Antibiotic sensitivity(Kirby bauer test)or (disc diffusion test)

Susceptibility Tests Agar diffusion  Kirby-Bauer Disk Diffusion Test (cont’d) Agar diffusion  Kirby-Bauer Disk Diffusion Test

Five Basic Mechanisms of Antibiotic Action against Bacterial Cells: Inhibition of Cell Wall Synthesis (most common mechanism) Inhibition of Protein Synthesis (Translation) (second largest class) Alteration of Cell Membranes Inhibition of Nucleic Acid Synthesis Antimetabolite Activity(folic acid synthesis)

1. CELL WALL SYNTHESIS INHIBITORS β-Lactam Antibiotics Penicillins Cephalosporins Penicillins penicillin, oxacillin ampicillin Cephalosporins & Cephamycins cefaclor cefoxitin cephalosporin Carbapenems Imipenem Monobactams moncyclic (different from other b-lactams) Axtreonam

CELL WALL SYNTHESIS INHIBITORS Action of β-Lactam antibiotics The ß-lactam antibiotics combine with and inactivate the tranpeptidase: they are the targets of β-lactam antibiotics and are named penicillin-binding protein (PBPs). Blocks cross-linkage of peptidoglycan

Mechanism of Action INHIBITION OF PROTEIN SYNTHESIS Tetracyclines bind to 30S subunit and interferes with the attachment of the tRNA carrying amino acids to the ribosome effective against: Chlamydia Rickettsia Mycoplasma Brucella Mostly bacteriostatic broad spectrum Active against obligate intracellular bacteria and cell wall deficient organisms chelated by divalent cations and their absorption and activity are reduced, so shouldn’t be taken with dairy products or many antacids used in otis medea and sinusitis b/c usually effective, but also given in viral cases, which is bad. Probably no antibiotic group has been oversubscribed more. Doxycyline - longer acting tetracyline

3. INHIBITION OF DNA/RNA SYNTHESIS Inhibitors of RNA Synthesis: Selectivity due to differences between prokaryotic and eukaryotic RNA polymerase: (Rifampcin) Inhibitors of DNA Synthesis: DNA gyrase use along with other drugs for TB

Origin of Drug Resistance Non-genetic metabolic inactivity Mycobacteria non-genetic loss of target penicillin – non-growing cells, L-forms intrinsic resistance some species naturally insensitive

Origin of Drug Resistance Genetic spontaneous mutation Chromosomal Resistance Extrachromosomal Resistance (Plasmids) Spontaneous evolution occurs at low rate (~1 in 10 million cells) Grow up 10 to the 9 or tenth cells, there is a good chance one cell is resistant to Streptomycin due to mutation, plate the load, isolate the resistant. Streptomycin: binds to 30S subunit of ribsome causes distortion and misreading Single target, easy to get spontaneuos mutation. Multiple targets are harder b/c several different mutations are required to prevent binding of the drug. Plasmids Transposons Integrons Thus if organism has two different plasmids, an antibiotic resistant gene can move from one to another. In this way, a gene could move from a narrow range host plasmid to a wide range host plasmid. Wide range host plasmids, in contrast to narrow range host plasmids, can replicate even if they are transferred to unrelated bacteria.